Behavior and properties of tin slag polyester polymer concrete confined with FRP composites under compression

2.2.1 Sieving/sieve analysis

In as-received form, tin-slag was sticky with a tendency of finer size to attach with coarser aggregates. A drying session at 105∘C lasting 2 hours was introduced for aggregate prior to be sieved by mechanical stirrer using 4 mm, 3.18 mm, 1.4 mm, 2 mm and 1-mm mesh sizes.

In accordance with a reported study seeking least void content, fineness modulus of three finer sizes i.e. 2 mm, 1.4 mm and 1 mm were calculated to determine their concentration in total mixture [16]. Sieve analysis in conformity with ASTM C136 / C136M - 14 was carried out to calculate fineness modulus of 3.71, 3.07 and 2.85 for 2 mm, 1.4 mm and 1 mm, respectively. The Fineness modulus is an empirical figure obtained by adding the total percentage of the sample of an aggregate retained on each of a specified series of sieves and dividing the sum by 100. A 200 g of aggregate was taken for analysis and three repetitions were carried out to take the average of them for final calculation.

2.2.2 Polymer concrete (PC) casting

Table 3 shows the values of fineness modulus along with percentage of coarse and fine aggregates for each of the group. Values of fineness modulus were put into Eq. (1) to get aggregate fraction finer than larger size fraction for each group.

Where

Y = Proportion of aggregate finer than larger size aggregate X = Fineness modulus

Specimen of 50mmdiameter and 100mmheightwere cast in PVC tubes, used as mold, by mixing pre-calculated proportion of resin, aggregate and initiator before pouring into mold. Curing was allowed for three days in a temperature range of 25-30∘C. For this study, one half of the coarser size aggregate was considered finer size aggregate. Aggregate to resin ratio was set at 70:30 percent by weight. Sample preparation was carried out under the guidelines of ASTM C 192/470.

2.2.3 Confinement

Carbon and glass fabrics of rectangular shapewere cut into defined dimensions of 100 mm width and 207 mm length. Circumference of the specimen was calculated to be 157 mm with an additional 50 mm for overlap. Epoxy resin was applied on carbon and glass fabric adopting wet layup process. Similarly, in another repetition, epoxy was applied on the surface of specimen. Next, epoxy impregnated carbon and glass fiber were wrapped on the specimen in hoop direction. An outlook of test specimen is provided in Figure 1. All of the confinement activity was carried out under the guidelines of American Concrete Institute guideline ACI 440.2R-08. Details of confinement plan is given in Table 4.

Figure 1

An outlook of confined samples

3.1.1 Stress-strain response

Figure 3 shows the mechanical response of tin-slag polyester polymer concrete under compression for all aggregate sizes. The basic response of three samples is identical with three distinct zones of curve i.e. ascending, semi-horizontal and post-peak descending.

Figure 3

Graph of stress versus strain of unconfined polymer concrete

First zone (Ascending) shows steady rate of increse in stress with associated strain. Initial short linear portion of curve is induced by adjustment of platen with the surface of specimen. This portion is marked by extra strain at low value of stress. At the end of it starts the consistent response of composite system where stresses are being distributed equally across the available area. Almost linear trend of ascending region in all samples confirms the effective interaction of heterogeneous elements within the composite system in providing structureal response.

Second zone (near-horizontal) starts where gradient, apparent in first zone, starts to diminsh to give way to horizontal semi-linearity. Stress in this zone remains near to ultimate strength value accompained by large degree of deformation. This behavior is induced by the gap-graded configuration of aggregate which renders the structure with least void content, thereby, evoking identical elastic deformation response from across the sample.

Third zone (post-peak descending) incorporates fracture driven response. After attaining maximum stress which is associated with fracture of specimen, there will be steady decline in stress value. At this stage, structure will fail to behave cohesively to the applied load and starts to release absorbed energy in the form of propagation of cracks. Also, there will be detachment of aggregates from matrix which is a result of interphase failure [17].

3.1.2 Mechanical properties

Figure 4 exhibits the compressive strength and modulus values for all aggregate size samples. With the increase in aggregate size for gap-graded configuration, compressive strength increases continuously with the highest value of 37.7MPa for AU. For gap-graded aggregates of coarsest size, this increment of compressive strength was attributed to effective distribution of finer size (2 mm) in granular skeleton because of two factors i.e. angular shape and higher concentration. As a principle, concentration of finer aggregates in gap-graded mixture increases with fineness modulus of finer aggregate. This will increase the likelihood of finer size getting place between coarser aggregates thus providing cushioning effect. Also, angular shape will reduce the segregation of aggregates during mixing and settling in curing stage. On the other hand, spherical particles that abounds in finer sizes have a tendecy to segregate during mixing at sample preparation stage.

Figure 4

Graph of PC compressive strength and modulus variation with aggregate size

Compressive modulus of PC increases with decrease in aggregate size. This phenomenon has been noticed in case of portland cement concrete when the aggregate size range adopted was 3 to 14 mm [18, 19]. The rationale for this increment in modulus is the volumetric increase in inter-phase which increases with the the availabilty of surface area. This relation has been reported in a study considering polymer cementitious compounds where modulus is considered to be the product of two phases: aggregate and interphase [19].

Figure 5 juxtaposes the compressive strength and modulus trends acquired across the three aggregate groups. The contrasting relation of strength and modulus is evident induced by variation in aggregate size.

Figure 5

Graph of comparison of compressive strength and modulus variation with aggregate size

3.2.1 Load-deformation behavior

Figure 6 shows the compressive behavior for GFRP and CFRP confined samples. Unconfined sample curves are inserted as reference. FRP confined PC showed typical bilinear curve but with sharp transition from core-induced to jacket-induced region in the curve. The convention of assuming response from concrete core in first linear region and from jacket in second linear region, in case of ordinary concrete, does not seem to hold true for polymer concrete.

Figure 6

Graphs of load versus deformation of groups A (4+2 mm), B (3.18+1.4 mm) and C (2+1 mm) confined with GFRP and CFRP (Black lines indicate extent of shared path of curves)

In general, curves for GFRP confined samples show identical behavior regardless of aggregate size. The bilinear trend is obvious but with differences in initial sections of the curve. For A1G and A2G, the inital response is same as that of unconfined sample AU. Same slope of the curves regrdless of confinement indicates that the granular skeleton of the strucutre is effecient enough to support the load untill it reaches around 54% (39 kN) of maximum load sustained by uncofined specimen. At this point, curves of the confined specimen start to diverge from the common path. This behavior does not match with that of confined portland cement concrete where curve region representing confinement emerges when the load is around peak load of unconfined concrete [20, 21, 22]. This results from the much higher value of modulus of elasticity of concrete (14-41 GPa) which attracts major part of the load in the first region of bilinear curve. For B1G, B2G, C1G and C2G, curves start to diverge almost immediately after the onset of load. This change of slope to higher position indicates that polymer concrete granular structure, itself unable to carry load, starts to transfer it to the jacket.

Even with the reduced capacity of polymer concrete to carry load with reduction in aggregate size, the role of concrete core is evident in the load-displacement response of confined specimen. Both, for A1G and A2G first linear region contains mixed reaction from both jacket and core, but second linear region is primarily influenced by pressure generated by jacket. This is causing declivity in slope at higher regions of the curves.

The typical two linear regions are noticeable in case of CFRP but with transition zone which is much sharper than GFRP confined sample curves. This is because of high stiffness of the carbon fiber composite which is sensitive to minute expansion of the concrete core. As the contribution from stiffer jacket to support load starts from the onset of experiment, the transfer of load to jacket, as principal load bearing component, becomes less noticeable. This fact has been verified in a study that reports mechanical properties of jacket and their thickness at forefront in giving performance of ordinary strength concrete (20 to 50MPa) [23].

When performance of one layer samples is compared with two layers, the effect of concrete core becomes evident especially for higher strength samples of group A. Group A is distinguished from group B and C in showing a greater matched region of curves (up to 86.24 kN) for one and two layers CFRP. However, for group B and C, this region of identical slope is reducing. For Group B, this region extends upto 62.72 kN; and for Group C, it resides below 30 kN. Such a behavior is dictated by the load bearing capacity of concrete core, which sees a declining trend from group A to C. This finding has also been reported in a study conducted on normal and high strength concrete wrapped with CFRP. Stress-strain curves for one, two, and three layers exhibit a longer region of close values of slope when strength of core increases from 32 MPa to 107 Mpa [24].

3.2.2 Mechanical properties

Figures 7 and 8 outline the compressive strength of samples and increment in strength with number of layers. Group A, in conformity with the results of unconfined samples, shows the highest degree of increase in strength both for carbon and glass fiber composites which can be appreciated by observing consistent slope with number of layers in Figure 8. This is precisely because of the same reason as of the unconfined sample i.e. granular skeleton is well distributed to support structural load. One layer of glass has increased the ultimate strength to about 84.7% whereas two layers have pushed it to 160.8% compared to the strength of unconfined sample (37.71 MPa). The same is true of carbon where increase in strength for one layer is 128.05% and for two layers, 231.66%.

Figure 7

Bar-chart of compressive strength of CFRP/GFRP confined polymer concrete

Figure 8

Graphs of compressive strength versus no. of layers for gorups A (4+2 mm), B (3.18+1.4 mm), and C (2+1 mm)

In group B, the percentage increment in strength for one and two layers of GFRP is 85.47% and 142.33%. This loss of effeciency can be glimpsed in Figure 8 (Group B). For CFRP, theses values are 122.26% and 191.03% for one and two layers. For group C, one layer of glass offers 105.26% increase whereas two layers give 169.69%. One ply of carbon gives 141.95% and two layers 260.26% increment in strength. The whole result spectrum has been summarized in Table 5.

Compressive strength increment within group B and C confined samples follow the same trend as that of group A; but across both groups, properties of samples occasionaly overlap or closely lying to each other. This happens because of reduction in aggregate size, values of fineness modulus of finer sizes (1.4 and 1 mm) lying close to each other, greater control of jacket on properties with reduction in core strength and reduced effeciency of gap-graded principle for finer sizes.

Figure 9 provides an outlook of increment in modulus with confinement for three group sizes. The dominent feature of bar-chart of modulus is ascending trend with the increase in number of layers for both CFRP and GFRP across groups. Summary of modulus values are given in Table 5.

Figure 9

Bar-chart of compressive modulus of group A, B and C confined samples

It is true in case of modulus, as in case of ultimate strength, that the lower the value of modulus of concrete core, higher will be the effectiveness of confinement in influencng ultimate value of modulus. This is evident in case of group A (Figure 10) which has lowest value of modulus 1.18 GPa. But, the values of confined samples are comparable/closer to group C samples that has highest concrete core modulus of 1.66 GPa.

Figure 10

Graphs of compressive modulus versus no. of layers for gorups A (4+2 mm), B (3.18+1.4 mm), and C (2+1 mm)

This finding that modulus values of confined samples do not increase substantially/proportionally as the modulus of control/unconfined sample increases, leads to the realization that FRP is controlling the ultimate values of modulus for the confined system. Seemingly, closely lying/overlapping values of modulus across groups is because of FRP properties with inconsequential effect from polymer concrete. The influence of polymer concrete is, also, difficult to analyze becasue of the slight increment in modulus value from 1.18 to 1.66 GPa across groups which can not translate into a meaningful distinctive performance.

Another fact testifies this conclusion, and that is stiffness of FRP composite. Carbon confined samples have higher percentage increment than glass that indicates role of FRP’s stiffness in controlling ultimate value of modulus for confined configuration (Figure 10).

Table 5 shows the fracture energy values of each sample. The values of fracture energy have been acquired by calculating area under load-displacement curve with a basic assumption that as soon as the maximum load reaches, specimen undergoes sudden failure [13]. Figure 11 presents variation in ductility indices with no. of layers for three confined groups. Ductility indices are a ratio of fracture energy values of confined to unconfined specimens.

Figure 11

Graphs of ductility indices versus no. of layers for gorups A (4+2 mm), B (3.18+1.4 mm), and C (2+1 mm)

It can be observed that fracture energy increases with the addition of FRP layers i.e. it can absorb more energy than unconfined sample before failure. For group A, ductility indices reach 157% and 156% for two layers of glass and carbon composites. These are the highest indices in all three groups partly becasue highest values of extention at maximum load recorded for these samples are i.e. 6.17 mm and 5.35 mm for GFRP and CFRP.

For one layer samples of both CFRP and GFRP of group A and C, results do not produce a clear trend. Group B represents lowest indices of all groups and types of fibers. That is because of factors affecting the area under curve such as low ultimate load value and displacement. Influence of these competing factors is appreciated by the fracture energy of unconfined samples AU, BU and CU. AU has the highest value of fracture enegy, primarily, owing to highest value of strength whereas BU (5.18 mm) and CU (4.77 mm) can undergo larger displacement than group A according to data.

It is logical to observe the better performance of GFRP confined polymer concrete in absorbing energy than CFRP. This comes from the materials properties of GFRP which has way low stiffness than CFRP. Also, ductility increment of GFRP confined sample shows competing (A2G) and better (A1G) perofrmance than CFRP confined counterparts in terms of absorbed energy although it has lower strength than CFRP.